1. Field of the Invention
The present invention relates to a surface acoustic wave device to be used as, for example, a resonator and a bandpass filter, a surface acoustic wave filter, and a manufacturing method for the surface acoustic wave device.
2. Description of the Related Art
In recent years, in portable telephone systems, with the increase of subscribers and diversified services, each of the frequency bands for transmission and reception has been broadened and the transmission frequencies and reception frequencies have come close to each other. Accordingly, in the bandpass filters to be used in portable telephones, it is required that the filters are broadband and that the attenuation characteristics in the vicinity of passbands are superior. When EGSM as a portable telephone system in Europe is taken as an example, the transmission side frequency band is 880 MHz to 915 MHz and the reception side frequency band is 925 MHz to 960 MHz.
Transmission signals become noise in reception side circuits. Accordingly, in the bandpass filters to be used in reception side circuits, it is necessary to make signals pass through in the band of 925 MHz to 960 MHz and to attenuate signals in the band of 880 MHz to 915 MHz. That is, the filtering characteristics having 925 MHz to 960 MHz as a passband and 880 MHz to 915 MHz as an attenuation band are required. As a result, although the pass bandwidth is required to have a broad band of 35 MHz, the frequency difference between the passband and the attenuation band is only 10 MHz.
On the other hand, in the surface acoustic wave filters used as a bandpass filter in the portable telephones, a 36xc2x0 LiTaO3 substrate is used. As for this substrate, a temperature dependence of frequency is as large as xe2x88x9230 ppm/xc2x0 C. to 35 ppm/xc2x0 C. As a result, it is necessary to provide a margin of temperature change in designing circuits including surface wave devices.
Furthermore, when frequency variations during manufacture are taken into consideration, the frequency spacing between the passband and the attenuation band becomes much narrower. Therefore, it is more important to increase the steepness of the filtering characteristic in the vicinity of the passband.
The bandwidth of a surface acoustic wave filter and the steepness in the vicinity of the passband are nearly uniquely fixed by the electromechanical coupling coefficient of a piezoelectric substrate. Generally, when the electromechanical coupling coefficient is large, broadband filtering characteristics can be obtained, and when the electromechanical coupling coefficient is small, filtering characteristics having superior steepness can be obtained.
Accordingly, when piezoelectric substrates having different electromechanical coupling coefficients are used according to elements in surface acoustic wave filters, steep and relatively broadband filtering characteristics can be obtained in accordance with these combinations.
In Japanese Unexamined Patent Application No. 7-283688, a method for adjusting electromechanical coupling coefficients is performed in such a way that in a surface acoustic wave filter of a ladder-type circuit construction, the surface wave propagation direction in a series-arm resonator is made different from that in a parallel-arm resonator. In a 36xc2x0 Y-cut LiTaO3 substrate, the electromechanical coupling coefficient is dependent on the surface wave propagation direction. When the surface wave propagation direction is supposed to be directed along the X axis, the electromechanical coupling coefficient becomes maximum, and when the surface wave propagation direction is deviated from the X axis, the electromechanical coupling coefficient becomes smaller. Accordingly, for example, in a surface acoustic wave filter having a ladder-type circuit construction, when the propagation direction in the series-arm resonator is directed along the X axis and the propagation axis in the parallel-arm resonator is deviated from the X axis, as shown in FIG. 14, the spacing between a resonant frequency and antiresonant frequency in the resonance characteristic A of the parallel-arm resonator is narrowed compared with the characteristic (shown by broken line Aa) of the case where the propagation direction of the parallel-arm resonator is not deviated from the X axis and compared with the resonance characteristic B of the series-arm resonator. Accordingly, a filtering characteristic having superior steepness can be obtained as shown by broken line in FIG. 15. Moreover, in FIG. 15, the solid line D shows the characteristic where the surface wave propagation direction is not different in the series-arm resonator and parallel-arm resonator.
On the other hand, in Japanese Unexamined Patent Application Publication No. 8-65089, a method for adding capacitance to each resonator in a surface acoustic wave filter of a ladder-type circuit construction is disclosed. When capacitance is added in parallel to a surface acoustic wave resonator, an antiresonant frequency is decreased and the spacing between a resonant frequency and antiresonant frequency is narrowed in the same way as in the case of the method disclosed in Japanese Unexamined Patent Application Publication No. 7-283688. Accordingly, it is supposed that a filtering characteristic having superior steepness can be obtained.
However, in the surface acoustic wave filters disclosed in Japanese Unexamined Patent Application Publication No. 7-283699 and Japanese Unexamined Patent Application Publication No. 8-65089, there were various problems.
For example, in the former, there was a problem that, when the propagation direction of a surface wave is deviated from the X axis in a 36xc2x0 Y-cut LiTaO3 substrate, the power flow angle as an angular difference between the direction of travel of the surface wave and the direction of the energy transfer increases, the leakage of energy from the waveguide increases, and the loss increases. Furthermore, since it is necessary to provide a plurality of surface acoustic wave resonators having different propagation directions on the same piezoelectric resonator, the size of a surface acoustic wave filter is greatly increased. Furthermore, there were cases that different resonators are acoustically partially coupled and because of the coupling the characteristics become deteriorated.
On the other hand, in the method described in Japanese Unexamined Patent Application Publication No. 8-65089, in order to add capacitance, the chip size was greatly increased. Furthermore, because the filtering characteristics cannot be changed after the electrodes have been formed, the frequency could not be adjusted.
In order to overcome the problems described above, preferred embodiments of the present invention provide a surface acoustic wave device in which the above-mentioned drawbacks of the prior art are solved and without causing a larger size, increased loss, and deteriorated characteristics, such that the steepness in the vicinity of the passband is superior when used as a bandpass filter, and provide a manufacturing method for such a surface acoustic wave device.
Other preferred embodiments of the present invention provide a surface acoustic wave filter which is characterized by low loss, excellent filtering characteristics, very small size, and superior steepness in the vicinity of the passband.
According to a preferred embodiment of the present invention, a surface acoustic wave device includes a piezoelectric substrate made of LiTaO3, and at least one interdigital transducer disposed on the substrate and having a plurality of electrode fingers. The polarization direction in at least one gap portion between electrode fingers of the at least one interdigital transducer is different from the polarization direction in other gap portions between electrode fingers on the same propagation path.
According to the unique structure and arrangement of this preferred embodiment of the present invention, the electromechanical coupling coefficient is greatly reduced compared with the case where the polarization direction of all the gap portions in the interdigital transducer is the same. Thus, a frequency spacing between a resonant frequency and an antiresonant frequency is greatly narrowed and, when used as a surface acoustic wave filter, the steepness in the vicinity of the passband is greatly increased.
In the surface acoustic wave device, the electrode fingers of the at least one interdigital transducer may be withdrawn so that the electromechanical coupling coefficient is reduced by the withdrawing. In this case, the spacing between a resonant frequency and an antiresonant frequency can be much more reduced, and, when used as a surface acoustic wave filter, the steepness of filtering characteristics in the vicinity of the passband can be much more increased.
According to another preferred embodiment of the present invention, a surface acoustic wave filter preferably includes a piezoelectric substrate, and a plurality of one-port surface acoustic wave elements each having an interdigital transducer which is disposed on the piezoelectric substrate. The plurality of one-port surface acoustic wave elements are arranged to define a ladder-type circuit having at least one parallel-arm resonator and at least one series-arm resonator. The polarization direction in at least one gap portion between electrode fingers in the interdigital transducer of one of the one-port surface acoustic wave elements is different from the polarization direction in other gap portions between the electrode fingers.
According to still another preferred embodiment of present invention, a surface acoustic wave resonator filter includes a piezoelectric substrate, and a plurality of interdigital transducers provided on the piezoelectric substrate. The polarization direction in at least one gap portion between electrode fingers in the interdigital transducer of one of the one-port surface acoustic wave elements is different from the polarization direction in other gap portions between the electrode fingers.
According to these unique structures and arrangements, the frequency spacing between a resonant frequency and an antiresonant frequency is narrowed in the at least one surface acoustic wave element. Accordingly, the steepness in the vicinity of the passband of the surface acoustic wave filter is greatly increased.
For example, in a parallel-arm resonator, when the polarization direction in at least one gap portion is different from the polarization direction in other gap portions as described above, the resonant frequency is increased in the parallel-arm resonator and the steepness of the filtering characteristic on the lower frequency side of the passband is increased. Furthermore, in a series-arm resonator, when the polarization direction in gap portions is different from each other as described above, the steepness on the higher frequency side of the passband of a surface acoustic wave filter is greatly increased.
In the filters, a portion of the electrode fingers of the at least one interdigital transducer may be withdrawn. Alternatively, the propagation direction of a surface acoustic wave in at least one of the interdigital transducers may be different from the propagation direction of other interdigital transducers.
In this case, the electromechanical coupling coefficient of the surface acoustic wave device is substantially decreased by the thinning-out process, and the steepness of filtering characteristics can be more effectively increased.
The manufacturing method for a surface acoustic wave device including a piezoelectric substrate and at least one interdigital transducer provided on the substrate and having a plurality of electrode fingers preferably includes the steps of forming at least one of the interdigital transducers on the piezoelectric substrate and applying a DC voltage so that an electric field strength of about 50 V/xcexcm or more is applied to the at least one interdigital transducer. In the case, frequency adjustment may also performed by applying the DC voltage.
For the purpose of illustrating the invention, there are shown in the drawings several forms that are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.
Other elements, features, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.